1. Field of the Invention
The present invention relates to an optical detector for optically detecting abnormal portions in periodic patterns formed on industrial products. More specifically, the present invention relates to an optical detector for detecting, for example, defects in, or foreign matter attached to circuit patterns formed on wafers or reticles during manufacture of semiconductors.
2. Description of the Related Art
According to an optical theory, when an object is irradiated with coherent light beam such as laser light, the light beam is diffracted and scattered on the object. When a lens receives the diffracted light beam and forms an image of the object on its image plane, a so-called Fourier transform image of the object is formed on a Fourier transform plane defined between the lens and the image plane. When the object is formed with a periodic pattern, the Fourier transform image of the object has a periodic image corresponding to the periodic pattern on the object. When the object has defects on the periodic pattern, the Fourier transform image of the object is formed from a combination of the periodic image corresponding to the periodic pattern on the object and a non-periodic random image corresponding to defects. Accordingly, locating, on the Fourier transform plane, a spatial filter for blocking the periodic image (i.e., the Fourier transform image of the periodic pattern) can separate an image of the defects from the image of the periodic pattern and form the defect image on the image plane.
FIG. 1 shows a conventional optical detector proposed by Japanese Patent Laid-Open Publication No. 5-45862 and employed with the above-described optical theory with the use of a liquid crystal shutter as the spatial filter.
The conventional optical detector has a laser source 102 for irradiating an object mounted on a table 101, such as a wafer formed with a periodic circuit pattern, with laser beam by a small incidend angle .theta.. A condenser lens 103 is provided for forming an image of the object onto its image plane, on which a charge-coupled device (CCD) sensor 106 is located for picking up the formed image and for generating an image signal. A defect detecting portion 107 receives the image signal to thereby detect defects formed on the circuit pattern on the object. A twisted nematic (TN) liquid crystal shutter 105 is located on a Fourier transform plane defined between the lens 103 and the CCD sensor 106. The TN liquid crystal shutter 105 includes a twisted nematic liquid crystal layer 105c sandwiched between a pair of electrode layers 105b and 105d, which are sandwiched between a pair of polarizers 105a and 105e. A pattern generator 108 is provided to apply electric voltages of desired patterns between the electrode layers 105b and 105d to thereby control the liquid crystal shutter 105 as a desired spatial filter. A half mirror 104 is located between the liquid crystal shutter 105 and the lens 103 for guiding a half of the light from the lens 103 to each of the liquid crystal shutter 105 and a television camera 110. The television camera 110 is located at a position for picking up a Fourier transformed image of the object reflected from the half mirror 104. A central processing unit (CPU) 109 is connected to the television camera 110, the pattern generator 108 and the defect detecting portion 107.
In order to detect defects on a desired wafer, a sample wafer having the same periodic circuit pattern with the desired wafer is first located on the table 101. The television camera 110 picks up the Fourier transformed image of the circuit pattern, and transfers information on this image to the CPU 109.
Then, the wafer desired to be detected is located on the table 101. The CPU 109 controls the pattern generator 108 to apply electric voltage of a pattern corresponding to the Fourier transformed image of the circuit pattern detected for the sample wafer. As a result, the liquid crystal shutter 105 operates as a spatial shutter for blocking the Fourier transformed image of the periodic circuit pattern formed on the wafer. When the laser source 102 is driven to irradiate the wafer with laser light, only the portion of the Fourier transformed image of the wafer corresponding to the defects formed thereon passes through the liquid crystal shutter 105 to reach the CCD sensor 106. As a result, the CCD image sensor 106 picks up the image of the defects formed on the wafer, which is then processed by the defect detecting portion 107.
However, because this conventional optical detector has a television camera 110, the entire structure of the optical detector becomes large. In addition, it is necessary to previously pick up the Fourier transformed image of the sample wafer, and therefore the processing is complicated.
FIG. 2 shows another conventional method for optically detecting defects in periodic patterns proposed by Y. Mitsuhashi, et al. in "Optical Inspection of Industrial Pattern Defect Using New Spatial Filter" (Journal of the Electro-technical Research, 43-7,8) published on 1979.
In this conventional method, an object 201 having periodic patterns formed with defects is located on a front focal plane of a Fourier transform lens 202. Laser beam radiated on the object 201 passes through the object and is Fourier transformed by the Fourier transform lens 202 to form a Fourier Transformed image of the object on a back focal plane of the Fourier transform lens 202. A spatial filter 203 made from a photographic film is located on the back focal plane of the Fourier transform lens 202 so as to block only a spectral component of the periodic patterns formed on the object. Accordingly, only the spectral component of the defects passes through the spatial filter 203. The spatial filter 203 is positioned on a front focal plane of an inverse Fourier transform lens 204 which is the same as the Fourier transform lens 202. The inverse Fourier transform lens 204 receives light having passed through the spatial filter 203 and subjects the light to inverse Fourier transformation. As a result, a real image of the defects formed on the object 201 is formed on a bad focal plane 205 of the inverse Fourier transform lens 204. Picking up the real image of the defects can detect the defects formed on the object 201.
Also in this conventional method, however, it is necessary to previously prepare the spatial filter 203 based on a sample object formed with the periodic pattern with no defects. Accordingly, this method is also complicated.